1. Field of the Invention
The present invention relates to a wavelength converting device for generating a secondary harmonic of incident light, which has a nonlinear optical crystalline waveguide utilizing a Second Harmonic Generation (SHG).
2. Description of the Related Art
In the wavelength converting device, it is necessary to satisfy the phase matching condition in order to perform effectively the SHG. For this, there are phase matching method such as an angle-tuning, a temperature-tuning and an electric-field-tuning which utilize birefringence of the substrate crystal in the wavelength converting device so as to satisfy the phase matching condition. Further, there are attempts using a mode dispersion in the waveguide such as a method for changing the film thickness, a method for providing an over-layer, a method for using the Cherenkov radiation or quasi-phase matching.
A ferroelectric having a large nonlinear optical constant such as LiTaO.sub.3 (hereinafter referred to as LT), LiNbO.sub.3 (hereinafter referred to as LN) or KTiOPO.sub.4 is preferably used for the material of the substrate of the wavelength converting device. A bulk crystal of LiNb.sub.1-x Ta.sub.x O.sub.2 (0.ltoreq.x.ltoreq.1) (hereinafter referred to as LNT), for example, is produced by the CZ method or TSSG method in which the crystal is pulled up from its fused fluid or the micro crystal manufacturing method in which the crystal is pulled up through a nozzle. However, these methods have problems in a high quality and high homogeneity of the resultant crystal, and difficulties for enlarging the crystal and forming its waveguide shape. Thus, it is attempted that a LNT crystal thin film is formed on a substrate such as sapphire in a sputtering method.
In addition, there is had been developed a plasma-vapor-phase-growth method in which a single crystal film or LNT is deposited on the sapphire substrate (Japanese Patent Kokoku No. 5-11078). According to this method, Li, Ta and Nb are oxidized in an oxygen plasma and then a LNT single crystal film or epitaxial growth is preformed on the sapphire substrate.
LNT is a ferroelectric having a high melting point and a high Curie point. There is known that LNT has an electromechanical coupling coefficient larger than those or other ferroelectrics. Furthermore, since LNT has a high nonlinear optical constant, it is preferably used for a wavelength converting device. However, there is a demand for a ferroelectric having higher properties excellent to those above mentioned performances in order to achieve an effective waveguide in the wavelength converting device